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Date:	Tue, 18 Dec 2012 14:02:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] mm: cond_resched in tlb_flush_mmu to fix soft lockups
 on !CONFIG_PREEMPT

On Tue, 18 Dec 2012 17:11:28 +0100
Michal Hocko <mhocko@...e.cz> wrote:

> Since e303297 (mm: extended batches for generic mmu_gather) we are batching
> pages to be freed until either tlb_next_batch cannot allocate a new batch or we
> are done.
> 
> This works just fine most of the time but we can get in troubles with
> non-preemptible kernel (CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY) on
> large machines where too aggressive batching might lead to soft lockups during
> process exit path (exit_mmap) because there are no scheduling points down the
> free_pages_and_swap_cache path and so the freeing can take long enough to
> trigger the soft lockup.
> 
> The lockup is harmless except when the system is setup to panic on
> softlockup which is not that unusual.
> 
> The simplest way to work around this issue is to explicitly cond_resched per
> batch in tlb_flush_mmu (1020 pages on x86_64).
> 
> ...
>
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -239,6 +239,7 @@ void tlb_flush_mmu(struct mmu_gather *tlb)
>  	for (batch = &tlb->local; batch; batch = batch->next) {
>  		free_pages_and_swap_cache(batch->pages, batch->nr);
>  		batch->nr = 0;
> +		cond_resched();
>  	}
>  	tlb->active = &tlb->local;
>  }

tlb_flush_mmu() has a large number of callsites (or callsites which
call callers, etc), many in arch code.  It's not at all obvious that
tlb_flush_mmu() is never called from under spinlock?

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